Tag Used for Monitoring the Tire Pressure

ABSTRACT

A measuring method for measuring physical variables comprises—the selection of a working point (AP) lying within a total measurement range (G) of a physical variable (M) to be measured,—the detection of a measured value (M(t1)) of the physical variable at a first measuring time (t1),—the determination of a displacement value (V(t1)) as the result of a subtraction of the measured value (M(t1)) measured at the first measuring time from the working point (AP),—the formation of change values (C(t2), C(t3) . . . C(tx)) of the physical variable (M) by acquiring subsequent measured values (M(t2), M(t3) . . . M(tx)) of the physical variable at subsequent measuring times (t2, t3 . . . tx) and addition of the displacement value (V(t1)) to the subsequent measured values.

FIELD OF THE INVENTION

The invention relates to a measuring method for measuring physicalvariables.

The invention relates furthermore to a measuring system having a sensorfor detecting a physical variable and for emitting measured values ofthe detected physical variable, and having an analog-digital converterfor detecting the measured values emitted by the sensor.

The invention relates furthermore to a data carrier.

BACKGROUND OF THE INVENTION

In the case of digital measuring methods and measuring systems, it isknown that electric power consumption and the time required formeasuring are dependent on the accuracy of the measurement, the productof electric power consumption and measuring time being relativelyconstant for a specific accuracy of measurement.

The use of such measuring methods and measuring systems presentsproblems in monitoring applications where relatively little energy isavailable, yet accurate measurements of a specific physical variable areto be made at regular intervals, as, for example, in the case oftire-pressure monitoring systems on motor vehicles.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to produce a measuring method of thekind specified in the first paragraph, a measuring system of the kindspecified in the second paragraph and a data carrier of the kindspecified in the third paragraph, wherein the above-mentioneddisadvantages are avoided.

To achieve the above-mentioned object, in a measuring method inaccordance with the invention, inventive features are provided so that amethod in accordance with the invention can be characterized in themanner specified hereinafter, namely:

Measuring method for measuring physical variables, comprising:

-   -   the selection of a working point lying within a total        measurement range of a physical variable to be measured,    -   the detection of a measured value of the physical variable at a        first measuring time,    -   the determination of a displacement value as the result of a        subtraction of the measured value measured at the first        measuring time from the working point,    -   the formation of change values of the physical variable by        acquiring subsequent measured values of the physical variable at        subsequent measuring times and addition of the displacement        value to the subsequent measured values.

To achieve the above-mentioned object, in a measuring system inaccordance with the invention inventive features are provided, so that asystem in accordance with the invention can be characterized in themanner specified hereinafter, namely:

Measuring system having a sensor for detecting a physical variable andfor emitting measured values of the detected physical variable, havingan analog-digital converter for detecting the measured values emitted bythe sensor and for converting the measurement signals into digitalmeasured values, having calculation means for determining a displacementvalue as the difference of a measured value measured at a firstmeasuring time from a selected working point lying within a totalmeasurement range of the physical variable, and having addition means,which are constructed to add the displacement value to the measuredvalues supplied to the analog-digital converter, so that as outputvalues the analog-digital converter supplies change values of thephysical variable that are formed from the acquired measured values plusthe displacement value.

To achieve the above-mentioned object, in the case of such a datacarrier a measuring system in accordance with the invention is provided,the data carrier comprising transmitting means for wireless transmissionof the change values and/or of measured values of the measured physicalvariable determined by the measuring system.

The features according to the invention enable monitoring of physicalvariables to be performed with substantially lower electric powerconsumption or shorter measuring time than according to the prior art,and the invention is therefore excellently suited for use in monitoringsystems in which little electrical supply energy is available. Theinvention is especially suitable for use in those monitoring systems inwhich a physical variable that changes slowly with time is to bepermanently monitored, such as, for example, the tire pressure in amotor vehicle tire. Furthermore, in many known monitoring systems, suchas, for example, tire pressure-monitoring systems, the physical variableis indeed measured at regular intervals, but what is really of interestafter a first recording of a measured value is the change in thisphysical variable over time, and not its absolute value. Through theinvention, the physical variable value to be measured is displaced intoa working range lying around a selected working point, the working rangeamounting to only a fraction of the total measurement range. Iftherefore, for example, the working range amounts to one quarter of thetotal measurement range, then for the same accuracy of measurement themeasuring time can shortened to up to a quarter of the originalmeasuring time or a corresponding reduction in the electric powerconsumption can be achieved. The invention can be very easily integratedinto contactless data carriers and thus offers wide-rangingopportunities for application.

It can be mentioned that a method and a system for monitoring tirepressure are known from the document US2003/0079536 A1; here, inaddition to the actual tire pressure, a parameter that influences thetire pressure is also measured, wherein on the basis of this parameteran optimum tire pressure is calculated, is compared with the tirepressure actually measured and, when the actual tire pressure differsfrom the optimum tire pressure by more than a predetermined value, avariation signal is issued. In contrast with the present invention, withthis known tire pressure-monitoring method and system it is always theactual tire pressure, that is to say, the complete measured value, thatis measured and a variation value that is calculated. No change value ismeasured.

The advantage obtained in accordance with the measures of claim 2 isthat, when the change value exceeds the selected working range, asimple, power-saving and time-saving adjustment of the displacementvalue can be performed, in order to bring the subsequent change valuesinto the working range again. The working range can therefore beselected to be small, which in turn contributes to saving of time andenergy.

The advantage obtained in accordance with the measures of claim 3 isthat, when the change value exceeds the selected working range, anaccurate re-calculation of the displacement value can be performed.

The advantage obtained in accordance with the features of claim 4 isthat the resolution for the detection of measured values of the physicalvariable and the resolution for the detection of change values can beset independently of one another, as required, for example, a higherresolution being set for the detection of change values in order tomonitor the change in the physical variable as exactly as possible.

The advantage obtained in accordance with the measures of claim 6 and 7is a hard-wired implementation combined with high accuracy and minimumexpenditure on components.

The advantage obtained in accordance with the measures of claim 8 isthat the resolution of the analog-digital converter and hence itsconversion time and its energy consumption are adaptable to theparticular requirements of the application. In particular, differentresolutions can thus be set in a simple manner, depending on whether theanalog-digital converter is being operated to measure the physicalvariable in the total measurement range or to detect a change value ofthe physical variable in the working range.

The advantage obtained in accordance with the measures of claim 9 isthat the resolution and the conversion time of sigma-delta converterscan be adjusted and their electric power consumption is low.

The advantage obtained in accordance with the measures of claim 10 isthat the measuring system according to the invention can be used as atire pressure-monitoring system that is superior to conventionaltire-pressure monitors in respect of electric power consumption andaccuracy.

The advantage obtained in accordance with the measures of claim 12 isthat the data carrier is operable in a frequency band, namely between300 and 900 MHz, in which many monitoring systems work, so that the datacarrier according to the invention can be integrated into thesemonitoring systems.

The advantage obtained in accordance with the measures of claim 13 isthat the data carrier does not require its own energy supply in the formof a battery or rechargeable battery, rather is supplied externally viathe received electromagnetic field, so that it is maintenance-free andcan be installed in a completely closed housing.

The advantage obtained in accordance with the measures of claim 14 isthat the data carrier can be supplied with electrical energy from thereceived electromagnetic field, but—unlike the case with passive datacarriers—even if the data carrier is removed from the range of theelectromagnetic field this energy is still available owing to thetemporary storage thereof.

These and other aspects of the invention are apparent from and will beelucidated, by way of non-limitative example, with reference to theembodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a time-based diagram to illustrate the measuring methodaccording to the invention.

FIG. 2 is a block diagram of a first embodiment of a measuring systemaccording to the invention.

FIG. 3 is a block diagram of a second embodiment of a measuring systemaccording to the invention that is integrated in a contactlesslyreadable data carrier.

DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a time-based diagram, by means of which the measuringmethod according to the invention is explained below. The time-baseddiagram shows the respective value of a physical variable M at differentmeasuring times t1 to t4. The total measurement range G, within whichall values of the physical variable M lie, shall be assumed in thisexemplary embodiment to be 128. The total measurement range G can dependon the measurement range of a sensor for detecting the physical variableor on a measurement range of an analog-digital converter connecteddownstream of the sensor. The first line in the time-based diagramrepresents the first measuring time t1, at which a measured value M(t1)of the physical variable M of 80 occurs. It should be mentioned that allnumerical data serve merely for explanation of the measuring methodaccording to the invention and are random. It should furthermore bementioned that for reasons of clarity the measured values over themeasurement range are not shown true to scale. Furthermore, within thetotal measurement range G a working point AP and a working range A isselected, the working range A begin defined by a lower limit value ALand by an upper limit value AH. The lower limit value AL is obtainedfrom the subtraction of a lower working range width a (here: a=20) fromthe working point AP. The upper limit value AH is obtained by additionof an upper working range width b (here: b=12) to the working point AP.It should be mentioned that the lower and the upper working range widthsa, b are selected to be the same width in most cases of application.Likewise, the working range widths a, b can be dynamically matched. Theworking range A lies between 0 and 32 and therefore amounts to onequarter of the total measurement range G. In accordance with themeasuring method of the invention, the measured value M(t1) acquired atthe first measuring time t1 is used to form a displacement value V(t1),by subtracting the measured value M(t1) from the working point AP:

V(t1)=AP−M(t1)=20−8=−60.

This displacement value V(t1) is used for the subsequent measurementsfor determining a change value. Thus, the second line in the timedependency diagram of FIG. 1 shows a second measuring time t2, at whicha measured value M(t2) of the physical variable M of 90 is acquired. Thechange value C(t2) can be calculated according to the invention byadding the displacement value V(t1) acquired at the first measuring timet1 to the measured value M(t2), that is:

C(t2)=M(t2)+V(t1)=90+(−60)=30.

The acquired change value C(t2) lies in the working range A, that is,within the limit values AL and AH. The advantage of the measuring methodaccording to the invention is that, after the first measurement, atwhich a measured value that can lie anywhere within the totalmeasurement range G is acquired, only the change values have to beaccurately detected, but these lie in a substantially reducedmeasurement range compared with the total measurement range G because ofdisplacement of the respective measured value into the working range, inthe present example within the lowermost quarter of the totalmeasurement range. An analog-digital converter that samples ameasurement range requires so much the less electric power and/ormeasuring time the smaller is the measurement range it has to sample. Asa rule of thumb, one can assume that the product of electric powerconsumption and measuring time for a specific value to be sampled isconstant and increases linearly with the magnitude of the measuredvalues. If, as in the present case, the values to be sampled by ananalog-digital converter, that is, the change values, lie only in thelowest quarter of the total measurement range, then one can anticipatean average reduction in electric power consumption to a quarter.

The case where the measured values change so significantly that thechange values C(tx) displaced by the displacement value V(t1) lieoutside the preferred working range A requires special treatment. Thiscase is shown in the third line of the time-based diagram of FIG. 1,which shows the measuring time t3. The measured value M(t3) appearing atmeasuring time t3 is 110. If the displacement value V(t1) of −60 isadded thereto, a change value C(t3)=50 is obtained. This value liesabove the upper limit value AH of 32 and hence outside the working rangeA. In other words, the change value C(t3) exceeds the working point APby more than the upper working range width b. So that during subsequentmeasurements the change value again lies in the working range, thedisplacement value must be corrected. According to the invention, twoapproaches are provided for this. Firstly, the measured value M(t3),which provided the basis for the calculation of the change value C(t3),or alternatively a subsequent measured value (Mtx), can be subtractedfrom the working point AP and the difference value resulting from thiscalculation can be used as new displacement value V(t3) respectivelyV(tx) for subsequent calculations of change values. This procedurecorresponds to that carried out at the time t1 in the case of the firstmeasured value acquisition. An alternative approach for determining acorrected displacement value V comprises adding a variation value X tothe previous displacement value. The variation value X should beselected to be at least sufficiently large that subsequent change valuesC will lie with great probability within the working range A again,wherein it should be mentioned that the measuring method according tothe invention is preferably used for monitoring slowly changing physicalvariables, so that from the knowledge of the instantaneous measuredvalues or the measured value trend in the past, a good estimation of thetendency of the subsequent measured values is possible, so that again bysimple mathematical or statistical methods a suitable variation valuecan be determined. The variation value X can also be changed in severalsteps in order to bring the subsequent change values C into the workingrange again without provoking sudden changes in the change values C. Ina preferred embodiment of this approach, as illustrated in the timedependency diagram of FIG. 1 at the time t3, a variation value X(t3) iscalculated by subtracting the change value C(t3) lying outside theworking range A from the working point AP, that is:

X(t3)=AP−C(t3)=20−50=−30

and the variation value X(t3) is added to the previous displacementvalue V(t1) producing the new displacement value V(t3):

V(t3)=V(t1)+X(t3)=−60+(−30)=−90.

The new displacement value V(t3) is used to determine the subsequentchange values, as illustrated in the time-based diagram at the measuringtime t4. It is apparent that owing to the displacement value V(t3) nowapplied, a measured value M(t4) of 110, unchanged compared with themeasuring time t3, leads to a change value C(t4) of 20 that lies withinthe working range A.

FIG. 2 illustrates a block diagram of a first embodiment of a measuringsystem 1 according to the invention, which is designed to execute themeasuring method according to the invention. The measuring system 1comprises a sensor 2, which supplies analog measured values (M(tx) ofthe sensed physical variable M as its output signal. The measured valuesM(tx) emitted by the sensor 2 are supplied to an input of an analogdigital converter ADC, which performs a digitization of the receivedsignals and supplies the digitized values to calculation means 3, whichare integrated in the analog-digital converter ADC. Furthermore, themeasuring system according to the invention is provided with additionmeans, which comprise a summation circuit 5 connected between the sensor2 and the analog-digital converter ADC as well as a digital-analogconverter DAC. The measured values M(tx) of the sensor 2 are fed to oneinput of the summation circuit 5 and the output voltage Van(tx) suppliedby the digital-analog converter DAC is fed to another input and thesignals present at the two inputs are added and supplied from an outputof the summation circuit 5 to the input of the analog-digital converterADC. Furthermore, the calculation means has an input 3 a for adjusting aworking point AP and a working range A. The mode of operation of themeasuring system 1 according to the invention is as follows: at a firstmeasuring time t1, for example on start-up of the measuring system or atregular intervals, the output of the digital-analog converter DAC is setto zero, so that the measured value M(t1) present at the summationcircuit 5 is passed on unchanged to the analog-digital converter ADC andis digitized in the analog-digital converter ADC. From the digitizedmeasured value M(t1) the calculation means 3 calculate a displacementvalue V(t1), by subtracting the measured value M(t1) from the workingpoint AP predetermined via the input 3 a and storing the result of thissubtraction as displacement value V(t1) and at the same time supplyingit as input signal to the input of the digital-analog converter DAC,which converts the displacement value V(t1) into an analog electricalvoltage Van(t1) and feeds it to an input of the summation circuit 5. Thesummation circuit 5 now supplies a change signal C(tx) to theanalog-digital converter ADC, which signal corresponds to the sum of themeasured value M(tx) and the analog displacement value Van(t1). Thedigitized change signal C(tx) is output by the analog-digital converterADC at the output OUT for further use by a monitoring device, notillustrated. It should be mentioned that, in addition to the changesignal C(tx), the measured value M(t1) on which the determination of thedisplacement value (Vt1) is based, as well as the calculateddisplacement value V(t1), can also be output via the output OUT, inorder to allow the monitoring device, not illustrated, to evaluate thetrend of the physical variable comprehensively. In an alternativeconstruction of the measuring system 1, instead of the change signalC(tx) at the output OUT, a measured value reconstructed by thecalculation means 3 could be output, this value being calculated fromthe change value C(tx) less the displacement value V(t1).

As long as the change values C(tx) move within the working range A, thedisplacement value V(t1) is retained. If the change values C(tx) lieoutside the working range A, a recalculation of the displacement valueis performed according to the measuring method of the invention, as wasexplained above with reference to FIG. 1. Since, after the initialdetermination or, if applicable, re-adjustment of the displacementvalue, the summation circuit 5 sends to the analog-digital converter ADCin the measuring system 1 according to the invention only input signalsthat lie in a small range of the total measurement range, theanalog-digital converter ADC is able to carry out digitization of thesupplied signals with reduced power consumption and reduced conversiontime. In a preferred embodiment, the analog-digital converter ADC is inthe form of a sigma-delta converter and has an input for changing theresolution and the conversion time, so that both the resolution of thetotal measurement range and that of the selected working range areadjustable.

The measuring system 1 according to the invention is excellently suitedfor use in monitoring systems in which a relatively slowly changingphysical variable must be monitored or in which the change in a physicalvariable rather than its absolute value is important in the monitoring.The measuring system 1 according to the invention is of particularadvantage in all those applications in which little electrical energy isavailable for supply of the measuring system 1. Such applicationscomprise, for example, tire pressure-monitoring systems, where themeasuring system is built directly into a tire or a wheel rim and apressure sensor is used as sensor 2.

A block diagram of a second embodiment of a measuring system 1′according to the invention that is integrated in a contactlesslyreadable data carrier 10 is shown in FIG. 3. The measuring system 1′comprises a sensor 2, for example, a pressure or temperature sensor,which senses a physical variable M and sends measured values M(tx) to ananalog digital converter ADC, which digitizes the received measuredvalues and sends the digitized values to calculation means 3, which areintegrated in the analog-digital converter ADC. The calculation means 3have an input 3 a for adjusting a working point AP and a working rangeA. From the received measured values M(tx) and the working point AP, thecalculation means 3 calculate a displacement value V(tx) in accordancewith the measuring method of the invention. This displacement valueV(tx) is supplied to a controllable voltage source 4, which in responseto the displacement value produces a d.c. voltage that is supplied to anoffset voltage input OS of the analog digital converter ADC anddisplaces the quantization range of the analog-digital converter ADC byan amount corresponding to the displacement value V(tx), so that at itsoutput OUT the analog-digital converter ADC delivers change valuesC(tx), which correspond to the addition of the measured value M(tx) andthe displacement value V(tx).

The change values C(tx) are supplied to transmitting means TRANS of anair interface 6, which comprises a coupling element 7, such as anantenna or coil, for transmitting and receiving electromagnetic signals.The transmitting means TRANS transmit the change values C(tx) in theform of electromagnetic signals via the coupling element to a readerstation 9, which evaluates the change values C(tx). In a preferredembodiment, the transmitting means TRANS operate in a UHF frequencyrange between 300 and 900 MHz. The frequency range of the transmittingmeans is not specifically restricted, however. Other frequency ranges inwhich the transmitting means are operable lie, for example, at 125 kHz,13.56 MHz or 2.4 GHz. The data carrier 10 and the measuring system 1′formed therein can be battery-powered. Here, preference is given to theuse of a secondary cell or a rechargeable energy storage mechanism 8 asbattery, which can be charged via energy from an electromagnetic field,by constructing in the air interface 6 receiving means REIC that receivean electromagnetic field built up by the reader station, extractingelectrical energy from this electromagnetic field and feeding it to theenergy storage mechanism 8 for charging and temporary storage, so thatit is available for power supply to all assemblies of the data carrier10. Alternatively, the data carrier 10 can be in the form of a passivedata carrier.

1. A measuring method for measuring physical variables, comprising: theselection of a working point lying within a total measurement range of aphysical variable to be measured, the detection of a measured value ofthe physical variable at a first measuring time, the determination of adisplacement value as the result of a subtraction of the measured valuemeasured at the first measuring time from the working point, theformation of change values of the physical variable by acquiringsubsequent measured values of the physical variable at subsequentmeasuring times and addition of the displacement value to the subsequentmeasured values.
 2. A measuring method as claimed in claim 1, wherein,if the change value exceeds the working point by more than apredetermined upper working range width or falls below the working pointby more than a predetermined lower working range width, a newdisplacement value is determined by adding a variation value to theprevious displacement value, the variation value preferably beingcalculated by subtraction of the change value from the working point. 3.A measuring method as claimed in claim 1, wherein, if the change valueexceeds the working point by more than a predetermined upper workingrange width or falls below the working point by more than apredetermined lower working range width, a new displacement value, isdetermined as the result of a subtraction of the measured value,measured at this measuring time or at a subsequent measuring time fromthe working point.
 4. A measuring method as claimed in claim 1, whereinfor the total measurement range of the physical variable to be measuredan overall resolution of an analog-digital converter sampling themeasured values is set, by dividing the total measurement range into atotal number of quantization ranges, and a working range resolution ofthe digital-analog converter of a working range containing the workingpoint is set, by subdividing the working range into a number ofquantization ranges.
 5. A measuring system having a sensor for detectinga physical variable and for emitting measured values of the detectedphysical variable, having an analog-digital converter for detecting themeasured values emitted by the sensor and for converting them intodigital measured values, having calculation means for determining adisplacement value as the difference of a measured value measured at afirst measuring time from a selected working point lying within a totalmeasurement range of the physical variable, and having addition means,which are constructed to add the displacement value to the measuredvalues supplied to the analog-digital converter, so that as outputvalues the analog-digital converter supplies change values of thephysical variable, which are formed from the detected measured valuesplus the displacement value.
 6. A measuring system as claimed in claim5, wherein the addition means include a digital-analog converter,controlled by the calculation means and of which the output signalrepresents the displacement value, and a summation circuit, to which themeasured values of the sensor and the output signal of thedigital-analog converter are suppliable for summation.
 7. A measuringsystem as claimed in claim 5, wherein the analog-digital converter hasan offset voltage input for adjusting a displacement of a zero point ofthe digitization range of the analog-digital converter and the additionmeans comprise a controllable voltage source connected to the offsetvoltage input, which voltage source is adjustable by the calculationmeans to a voltage representing the displacement value.
 8. A measuringsystem as claimed in claim 5, wherein the analog-digital converter has acontrol input for setting the resolution.
 9. A measuring system asclaimed in claim 5, wherein the analog-digital converter is in the formof a sigma-delta converter.
 10. A measuring system as claimed in claim5, wherein the sensor is in the form of a pressure sensor and themeasuring system is in the form of a tire pressure-measuring system. 11.A data carrier for measuring physical variables, having a measuringsystem as claimed in claim 5, wherein the data carrier comprisestransmitting means for wireless transmission of the change values and/orof measured values of the measured physical variable determined by themeasuring system.
 12. A data carrier as claimed in claim 11, wherein thetransmitting means emit electromagnetic signals in the UHF band.
 13. Adata carrier as claimed in claim 11, wherein the data carrier comprisesreceiving means for receiving an electromagnetic field and forextracting electrical energy from the electromagnetic field forsupplying the data carrier with electrical energy.
 14. A data carrier asclaimed in claim 13, wherein a chargeable battery in the form of asecondary cell, or a rechargeable energy storage mechanism, is provided,which are chargeable using energy from an electromagnetic field that isreceivable by the receiving means